The present invention relates to the manufacture of injection molded articles. Such articles can be molded from a polymer thermoset resin or can be composite articles, that is articles having a fiber reinforcement lattice within a cured resin matrix. More particularly, the invention relates to a method and apparatus for injection molding such polymer and composite articles.
Reaction injection molding and resin transfer molding are processes wherein dry fiber reinforcement plys (preforms) are loaded in a mold cavity whose surfaces define the ultimate configuration of the article to be fabricated, whereupon a flowable resin is injected, or vacuumed, under pressure into the mold cavity (mold plenum) thereby to produce the article, or to saturate/wet the fiber reinforcement preforms, where provided. After the resinated preforms are cured in the mold plenum, the finished article is removed from the mold.
The prior art teaches injection molding apparatus which consist of a pair of complementary or xe2x80x9cmatchedxe2x80x9d tools which provide these molding surfaces, which each tool being carefully machined, for example, from a rigid metal which is otherwise relatively nonreactive with respect to the resin to be used in conjunction therewith. Such matched metal molds are expensive to fabricate and are necessarily limited to the manufacture of a single article of a given design. Stated another way, even slight changes to the desired configuration of the article to be fabricated may necessitate the machining of an entirely new replacement tool.
Additionally, such known metal tools typically have substantial thermal mass which becomes increasingly problematic as the mold temperature deviates from the desired process temperatures. In response, such tools are often provided with an integral system of internal heating and/or cooling tubes or passages through which an externally supplied heating/cooling fluid may be circulated. However, in accordance with these prior art designs, the heating/cooling passages are positioned relative to the tool surfaces so as to leave a minimum spacing of perhaps 2 inches (5 cm) therebetween to ensure that the resulting article will be free of hot and cold lines or bands which might otherwise be generated in the article as a result of disparate heating/cooling rates during resin cure. This minimum spacing, in turn, inherently limits the ability of these prior art tools to accurately control temperature during the injection molding process, again, particularly where such processes are exothermic. And temperature control of the mold plenum becomes further problematic where variable-thickness articles are to be fabricated, given that the thicker portions of the article may well polymerize earlier, and will likely reach higher temperatures, than the thinner portions thereof.
Accordingly, such matched metal tools may have to be periodically idled for sufficient time to permit the mold to cool to an acceptable operating temperature, thereby substantially increasing the cost of article fabrication using such tools.
In an attempt to provide increased temperature control while facilitating removal of the finished article from the molding apparatus, the prior art teaches a modified molding apparatus wherein one of the mold surfaces is defined by a flexible member formed, for example, of rubber. The other mold surface is still defined by a rigid, thermally conductive metal tool which may be backed by a pressurized fluid such as steam whereby curing heat is transferred to the mold cavity for endothermic molding operations. Unfortunately, for such endothermic processes, heating but one side of the mold cavity may limit flexibility as to surface finish and other characteristics of the resulting article and, further, limit the degree to which resin cure may be accelerated. Moreover, where such molding apparatus are used in exothermic processes, the resulting heat accelerates deterioration of the flexible mold surface, thereby preventing long-term use of the tool. Moreover, such molding apparatus often requires evacuation of the mold plenum prior to injection of the resin therein, thereby rendering use and maintenance of such molding apparatus more complex, and processes employing such apparatus more time-intensive and costly.
Additionally, tools for closed molding applications are typically either machined or cast from expensive materials such as aluminum metals, composites, or metal alloys. There can be many problems with these kinds of tools, for example: high tooling costs, long lead times of 15 to 52 weeks, difficult or even impossible to modify and high repair or rebuild costs. What is needed, then, is a matched-tool injection molding apparatus featuring replaceable mold surfaces which are easier and less costly to fabricate than known rigid or flexible tools while further offering increased temperature control during both endothermic and exothermic processes thereby to provide articles of improved quality at lower cycle times.
Under the present invention, an injection molding apparatus includes a pair of mold sections, wherein each mold section itself includes a rigid housing and a semi-rigid membrane removably mounted to the housing so as to define a fluid-tight chamber therein. The membrane of each mold section, which, in turn, defines its molding surface, is preferably formed of an inexpensive composite material such as fiber reinforced plastics, or other suitable material; and, in accordance with the present invention, different membrane materials and/or characteristics may be selected for the respective membranes of each mold section. When the two mold sections are assembled with their respective molding surfaces in opposition to one another, a molding plenum is defined within which to fabricate the desired article. Thus, under the present invention, design changes to the article are readily accommodated through alteration or replacement of one or both of the low-cost membranes. Stated another way, under the present invention, a given mold section housing may be outfitted with any of a wide variety of relatively inexpensive composite membranes useful in the production of composite articles of different shapes, sizes and characteristics, thereby greatly reducing tooling costs as compared to the prior art. The dimensions of the mold section housings define a working envelope within which membranes of various different configurations can utilized. Any configuration designed to fit within the dimensions of the working envelope can be employed with no need to alter the mold section housing.
In accordance with the present invention, a noncompressible fluid is disposed within and fills the chamber of each mold section, whereby its respective membrane is supported so as to ensure proper dimensioning of the finished article while permitting slight dimensional flexing during resin injection, thereby to evenly distribute any injection-pressure loading of the membrane across its entire surface. The latter feature may prove especially advantageous where a spike in injection pressure is encountered during the resin injection step. As a further advantage, such slight dimensional flexing of the membrane during resin injection is believed to improve or enhance the flow of resin through the mold plenum. An expansion chamber in fluid communication with the chamber of one or both mold sections serves to accommodate thermal expansion of the membrane-backing fluid prior to injection of resin into the mold plenum, and subsequent to cure of the finished article, with a valve operating to isolate the chamber from the expansion chamber during resin injection and cure.
In another aspect of the invention, a composite sheet comprising a curable thermoset resin and reinforcing fiber is laid within the plenum of the molding apparatus. The mold sections forming the plenum are brought together to form a molded article. The temperature of the backing fluid is controlled to provide optimum cure temperature and cure rate for the reinforced composite sheet.
And, in accordance with another feature of the present invention, the backing fluid is itself preferably thermally conductive; and the molding apparatus further includes means in thermal communication with the backing fluid within one or both of the mold sections for regulating the temperature of the backing fluid. For example, in a preferred embodiment, the temperature regulating means includes a system of coils extending within each chamber, and an external heater/chiller unit of conventional design which is connected to the coil system and is operative to circulate a temperature control fluid at a predetermined temperature therethrough. In this manner, the temperature of the backing fluid and, correlatively, of the molding surface of each mold section may be closely regulated, thereby offering improved characteristics of the finished article and/or improved control of process parameters, such as cure time and temperature.
Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying drawings figures of certain presently preferred embodiments thereof.